Study on the Laser-EMAT Integrated System for Simultaneously Measuring the Width and Depth of Metal Plate

Abstract

A non-contact Laser-EMAT integrated system (LEIS) that uses a pulse laser as a generator and an Electromagnetic acoustic transducer (EMAT) as a detector is proposed. To explain the LEIS could simultaneously measure the width and the depth of metal plate, the finite element model (FEM) of the LEIS, which can describe the entire process from laser generates ultrasound to EMAT receives ultrasound, is established. In detail, the intensity ratios of Rayleigh wave, Shear wave and Longitudinal wave excited by laser under the consideration of three components, the particle displacement, the particle vibration velocity in out-plane direction and particle vibration velocity in in-plane direction, are focused. Combining the characteristics of the ratio and the EMAT as a velocity sensor, the Rayleigh wave and Longitudinal wave in the case of the particle vibration velocity in out-plane direction are chosen as the using waveforms, and the changes of directivity of various waveforms with laser parameters are studied. Furthermore, after the coil structure parameters and magnetic field direction of EMAT were reanalyzed and reconsidered, it is decided to use the coil with the wire spacing of 0.1 mm and a number of turns of 8 to combine with the transverse magnetic field to simultaneously detect the Rayleigh wave and Longitudinal wave generated by the laser. Finally, the signal-to-noise ratio of the two waveforms reach a balanced value, and can be clearly distinguished and extracted to achieve the purpose of measuring the depth and width of the metal plate at the same time.